1. Field of the Invention
The present invention relates to a fuel cell system provided with a fuel cell stack comprising a plurality of power-generating cells each having an electrolyte electrode assembly including an electrolyte interposed between an anode electrode and a cathode electrode, the electrolyte electrode assembly being interposed between separators.
2. Description of the Related Art
The fuel cell such as the solid polymer type fuel cell (SPFC) adopts an electrolyte membrane composed of a polymer ion exchange membrane (cation exchange membrane). The fuel cell comprises power-generating cells each including a membrane electrode assembly (MEA) interposed between separators (bipolar plates), the membrane electrode assembly including an anode electrode and a cathode electrode each principally containing carbon provided opposingly on both sides of the ion exchange membrane. Usually, the fuel cell is used as a fuel cell stack in which a predetermined number of the power-generating cells are stacked and integrally held.
In the above fuel cell stack, a fuel gas such as a gas principally containing hydrogen (hydrogen-containing gas), which is supplied to the anode electrode, contains hydrogen which is ionized into ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte. The electron generated in this process is extracted for an external circuit and is utilized as DC electric energy. An oxygen-containing gas such as a gas principally containing oxygen or air (oxygen-containing gas) is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode, and thus water is produced.
In the fuel cell which uses an electrolyte layer containing water impregnated in a polymer ion exchange membrane such as Nafion 112 (produced by Du Pond), the hydrogen ion conductivity in the electrolyte layer greatly depends on the water content of the electrolyte layer. In the above case, it is necessary to hold liquid water in the electrolyte layer. Therefore, it is impossible to set the power generation temperature (operation temperature) of the fuel cell to be not less than the boiling point of liquid water. Usually, the power generation temperature is controlled to be not more than 80xc2x0 C. to 90xc2x0 C.
A variety of cooling structures are used in order to control the power-generating cell to be at a predetermined power generation temperature as described above. For example, one of the cooling structures is of a circulating type in which deionized water or pure water or a mixture liquid composed of deionized water or pure water and ethylene glycol is used as a cooling medium. The cooling medium is introduced into the fuel cell stack to cool the power-generating cell. Next, the cooling medium is cooled to a predetermined temperature by performing heat exchange with a radiator or the like provided at the outside of the fuel cell stack. The cooling medium is supplied again into the fuel cell stack in a circulating manner.
However, in the above-described fuel cell stack, the power generation temperature is set to be not more than 80xc2x0 C. to 90xc2x0 C. Therefore, the temperature of the cooling medium discharged from the fuel cell stack, i.e., the temperature on the side of the cooling medium inlet of the radiator is lower than 80xc2x0 C. to 90xc2x0 C. By contrast, the temperature of the atmospheric air supplied to the radiator is about 40xc2x0 C. Therefore, the difference between the temperature of the cooling medium and the temperature of the atmospheric air is small, i.e., not more than 40xc2x0 C. to 50xc2x0 C.
Under these circumstances, it is necessary to use a considerably large radiator to effectively cool the cooling medium. When the maximum output of the fuel cell stack is maintained, it is specifically necessary to greatly increase a capacity of the radiator in comparison with a currently used radiator for an automobile of an internal combustion engine type. Then, there is a problem that the size of the entire equipment becomes inevitably large and it is considerably difficult to establish a layout when the fuel cell stack is incorporated into a body of an automobile.
It is a principal object of the present invention to provide a fuel cell system which makes it possible to effectively maintain high output operation and which makes it possible to miniaturize the fuel cell system with ease.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.